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The page below is a sample from the LabCE course Drug Testing Methods in the Clinical Toxicology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Gas Chromatography/Mass Spectrometry (GC/MS)

GC/MS instruments have been used by forensics laboratories for years to confirm positive screens but are a relatively recent arrival to the clinical laboratory scene. GC/MS combines separation of compounds by chromatography and structural identification by mass spectrometry.
Sample preparation
A preparation step or extraction is usually involved prior to analysis.
Purposes for sample preparation:
  • To remove undesired materials in the sample matrix.
  • To make the analytes more volatile and less polar by derivatization so they are more amenable to gas chromatography.
  • To convert the glucuronide metabolite back to the unconjugated parent drug by hydrolysis prior to analysis.
  • To increase sensitivity by concentrating the extract.
Types of extraction techniques used in sample preparation:
  • Precipitation reactions - Proteins are precipitated out of the sample upon the addition of a salt or organic solvent, or by changing the pH of the solution.
  • Liquid/liquid extraction (LLE) - Partitioning of solutes between two immiscible liquid phases. An aqueous liquid phase containing the analytes and another liquid phase, usually an organic solvent, are mixed in a separatory funnel. The sample analytes are transferred from the aqueous phase to the organic solvent. The organic solvent is drained into a container and evaporated to concentrate the analytes.
  • Solid phase extraction (SPE) - Partitioning of analytes between a liquid phase and a solid sorbent. The general procedure is to load a solution onto the solid phase trapping the analytes onto the sorbent, washing away undesired components, and then eluting the desired analytes with another solvent into a collection tube.
  • Solid supported liquid extraction (SLE) - An aqueous phase containing the analytes is poured onto an inert diatomaceous earth support contained within a cartridge and allowed to drain. An organic solvent is passed through the cartridge extracting the analytes into a collection tube.